Field device and method for calibrating a field device

ABSTRACT

The invention relates to a field device and a method for calibrating a field device, having a field device electronics and a sensor unit for process measurements, wherein the field device electronics receives measurement signals of the sensor unit, wherein the field device electronics includes an evaluation unit for evaluating the measurement signals and means for calibrating the field device. According to the invention, the means for calibrating the field device includes a digital adjusting element and a microprocessor, wherein the digital adjusting element is driven by the microprocessor for calibrating the field device.

The invention relates to a field device having a field deviceelectronics and a sensor unit for process measurements, according to thepreamble of claim 1, and to a method for calibrating a field device,according to the preamble of claim 12.

In the case of field devices having a field device electronics andconductive or capacitive sensor units of the assignee, it is necessaryin the case of extreme field conditions to conduct a calibration of thesensitivity of the field device using a potentiometer, whereinespecially a switching threshold of an evaluation electronics isadjusted using the potentiometer. The direction in which the switchingthreshold is changed depends on whether the field device is beingoperated in a “MIN” type of operation as pump protection, i.e. warningwhen a predetermined fill level is subceeded, i.e. fallen below, or in a“MAX” type of operation as overflow protection, i.e. warning when apredetermined fill level is exceeded. The calibration is then alwaysperformed, when an indicated sensor state, due to field conditions(accretion, etc.), does not agree with the actual sensor state.

In the case of practically all previously marketed sensor units for filllevel determination in liquids and bulk goods, such as work on the basisof capacitance measurements or conductivity measurements, sinusoidalelectrical alternating voltage signals are used as drive signals for thesensor units. Concurrently, the alternating signals also serve directlyas measurement signals. These alternating signals are normally producedby means of an analog oscillator and, for further processing, analogfiltered, rectified, and, in the case of limit level switches, comparedby means of analog comparators with predetermined threshold values.Microprocessors are, as a rule, only used to linearize, scale, andprovide the signals, prepared by means of analog electronics, with timedelays, switching hystereses, or inversions.

An object of the invention is to provide a field device which can becalibrated easily, especially also in the case where the field device isbuilt with a hermetically sealed housing, and to provide a method forcalibrating the field device.

The object is achieved according to the invention, with reference to thefield device, by the features of claim 1 and, with reference to themethod, by the features of claim 12. The dependent claims concernadvantageous embodiments and further developments of the invention.

A main idea of the invention is to provide a means in the form of adigital adjusting element for calibrating the field device, with theelement being driven by a microprocessor. The idea of the invention isimplemented especially advantageously, when a microprocessor is alreadypart of the field device electronics, since such microprocessor can beused for the calibration.

The calibration using a microprocessor and a digital adjusting elementis especially advantageous for field devices whose housings arehermetically sealed, so that no access is possible for a manual,trimming potentiometer.

In one advantageous embodiment of the invention, the digital adjustingelement acts by suitable signals on the evaluation unit and/or on asignal producing component in the field device, wherein the action ofthe digital adjusting element is, for example, to adjust a thresholdvalue in the evaluation unit or an amplification factor in the signalproducing component.

In an additional embodiment of the invention, the evaluation unitincludes a comparator, which can be embodied as an analog circuit or asa programmed function block, and produces a condition signal on thebasis of a comparison of the measured signal with a desired value, withthe desired value being produced by the microprocessor by way of thedigital adjusting element.

In an advantageous further development of the invention, the calibrationis initiated by a switching element, which is actuated contactlesslyfrom outside of the field device housing. The actuation occurs in thisfurther development, for example, by changing a magnetic field. Theswitching element is, for example, embodied as a reed relay or as a Hallsensor, and the associated actuation element as a permanent magnet. Thisembodiment has the advantage, that it can be implemented at favorablecost.

For protecting against accidental start-up of the calibration process, atime window is provided, within which a predetermined actuationprocedure, for example actuating twice, or actuating for a certainlength of time, must be carried out, in order to initiate thecalibration process.

In another further development of the invention, the interface betweenthe actuating element and the switching element is embodied as atransmitting/receiving element for a wireless data exchange ofinductive, optical or electromagnetic signals between the actuatingelement and the switching element, wherein the actuating element can beembodied as a memory card.

Thus, the interface between switching element and actuating element can,for example, be embodied as a transponder arrangement for inductive datatransfer using a transmitting coil and a receiving coil, wherein theactuating element includes the transmitting coil and the switchingelement includes the receiving coil.

Additionally, the interface can be embodied as an infrared interface foroptical transmission or as a radio interface, for example using theBlue-Tooth protocol, with the transmitting unit being part of theactuating element and the receiving unit part of the switching element.

If the switching element for initiating the calibration process isactuated over an air interface, it is advantageously possible toinitiate different calibration processes by differently coded signals,and/or to make the initiation dependent on a certain code, so that nounauthorized manipulation or accidental start-up of the calibrationprocess is possible.

The method of the invention for calibrating the field device includes,in a first method step, the determining of an operation type in whichthe field device is being operated. In an especially advantageousembodiment of the method, the field device electronics determines theoperating type by evaluating the connector assignments of the fielddevice.

The field device can be operated in a first operation type “MIN” as pumpprotection, i.e. a warning occurs, when a predetermined fill level issubceeded, or in an operation type “MAX” as overflow protection, i.e. awarning occurs, when a predetermined fill level is exceeded.

In a second method step, the field device electronics determines adesired state of the sensor, with the desired state being determined bymeans of a logical coupling between the signal representing theoperation type of the field device and the signal representing thestarting state of the condition signal and the fact that a calibrationprocess was started.

In the determining of the desired state of the sensor, it is assumedthat the actual state of the sensor is not represented by the startingstate of the condition signal, since a calibration process was startedmanually from outside the field device, so that the starting state ofthe condition signal must be changed such that the desired state of thecondition signal establishes itself, to represent the actual conditionof the sensor.

A calibration process is always initiated, when an indicated sensorstate corresponding to the starting state of a condition signal, due tothe field conditions (accretion, etc.), does not agree with the actualsensor state.

For protecting against accidental start-up of the calibration process,one embodiment of the method requires that the actuating element actuatethe switching element twice within a predetermined time span, in orderto start the calibration process. Of course, other actuation scenariosare conceivable for starting the calibration process, for example anactuation of the switching element by the actuating element for apredetermined period of time.

Depending on the determined type of operation and the determined desiredstate of the condition signal, a sensitivity of the field device ischanged in a third method step until a toggling of the condition signalis noted.

The sensitivity of the field device is increased, for example bydecreasing a threshold value for the comparator, or an amplificationfactor is increased in the case of producing the measurement signal inthe signal producing component. Analogously, the sensitivity of thefield device is decreased by increasing the threshold value for thecomparator or by decreasing the amplification factor of the measurementsignal.

The invention will now be explained in greater detail on the basis ofthe drawings, whose figures show as follows:

FIG. 1: Block circuit diagram of a first embodiment of the field deviceof the invention;

FIG. 2: Block circuit diagram of a second embodiment of the field deviceof the invention;

FIG. 3: Flow chart of the method of the invention.

As can be seen in FIG. 1, the field device 1 includes a field deviceelectronics 2 and a sensor unit 5 for determining and/or monitoring afill level of a medium in a container (not shown), with the sensor unit5 being embodied, for example, as a capacitive or conductive probe. Thefield device electronics 2 includes a microprocessor 3, an evaluationunit 4, a signal producing unit 6, a memory unit 7, a digital adjustingelement 11 and a switching element 9, with an actuating element 8actuating the switching element 9 contactlessly from outside of thehousing 14 of the field device electronics. In the illustratedembodiment, the housing 14 is hermetically closed, and the contactlessactuation occurs by changing a magnetic field, with the switchingelement 9 being, for example, a reed relay or a hall sensor, and theassociated actuating element 8 a permanent magnet. A interface 9.1 isprovided in the illustrated embodiment, such that a certain distance ofthe switching element 8 to the housing 14 is not exceeded. The interface9.1 can, however, be embodied as something more complex, such as, forexample, a transmitting/receiving unit for a wireless data exchange ofinductive, optical or electromagnetic signals between the actuatingelement 8 and the switching element 9. The illustrated evaluation unit 4includes a comparator 12 and a unit 13 for producing an output signal,with the comparator 12 being embodied as an analog circuit and producinga condition signal Z1 as a function of a comparison of a measurementsignal M1, produced by the signal producing unit, with a desired valueS1 produced by the digital adjusting element, with the desired value S1being produced by the microprocessor by way of the digital adjustingelement. The condition signal Z1 can assume two states and representseither the condition “free”, i.e. the sensor is not covered by a medium,or the condition “covered”, i.e. the sensor is covered by a medium.

Depending on the actual state of the condition signal Z1, the unit 13for producing an output signal produces a corresponding output signal,with the unit 13 for producing an output signal performing a requiredconditioning of the output signal for forwarding to a superordinatedunit. The output signal produced depends on the further use of theoutput signal, respectively on the transmission protocol being used.Thus, for example, a 4-20 mA-signal, a 0-10V-signal, a PFM-signal (pulsefrequency modulation signal), a binary switching signal, or a digitalcode, etc. can be produced. It is, however, conceivable that a pluralityof output signals (4-20 mA, 0-10V, PFM signal, binary switching signal,etc.) for different transmission protocols, respectively applicationpurposes, be produced and output.

The digital adjusting element 11 is, for example, a digital/analogconverter, which converts the digital, desired value from themicroprocessor 3 into an analog desired value S1. For calibrating thefield device 1, the microprocessor executes a calibration function 10,with the calibration function 10 being stored in the memory unit 7 as aprogram which can be run in the microprocessor 3. As part of thecalibration function 10, the desired value S1 is changed by way of thedigital adjusting element 11 and stored in a desired value memory 10.1.It is, however, also possible to change an amplification factor (dashedline in FIG. 1) in the signal producing unit 6. The changing of thedesired value depends on the state of the condition signal and on thetype of operation in which the field device is being operated. The typeof operation of the field device depends on the hookup of the sensorunit 5 with the field device electronics 2, i.e. how the sensor unit 5is connected with the signal producing unit 6. The field device can beoperated in an operation type “MIN” as pump protection, i.e. warningwhen a predetermined fill level is subceeded, or in an operation type“MAX” as overflow protection, i.e. warning when a predetermined filllevel is exceeded.

FIG. 2 shows, by way of example, as a second embodiment, a variation ofthe embodiment of FIG. 1. Different compared to the first embodiment isthat the evaluation unit 4 with comparator 12 and unit 13 for producingan output signal 13 and the digital adjusting element 11 areadditionally implemented as functions executable by the microprocessor,with the associated, executable programs likewise being stored in thememory unit 7. Present as a new unit is an analog/digital converter 15,which converts the analog measurement signal produced in the signalproducing unit 6 into a digital measurement signal M1 for processing inthe microprocessor. For the manner in which the individual unitsfunction, reference is made to the descriptions of FIG. 1.

As shown in FIG. 3, following a manually initiated start 100 of thecalibration process, the type of operation of the field device isdetermined 200. Depending on the determined type of operation (MIN orMAX), the method branches, with both branches then determining 300 thestarting state of the condition signal Z1. Depending on the determinedstarting state of the condition signal Z1, the sensitivity of the fielddevice is next increased 400, or decreased 500, in a loop, and the newvalue stored. If in a query 600 it is noted that the desired state ofthe condition signal Z1 has been reached, then the calibration processis ended 700. If the desired value is not yet reached, then thesensitivity is again changed 400, 500, until the desired state of thecondition signal is reached. For changing the sensitivity, the desiredvalue S1 of the comparator and/or the amplification factor for themeasurement signal M1 is changed, with the desired value S1 beinglessened, respectively the amplification factor increased, forincreasing the sensitivity, and with the desired value S1 beingincreased, respectively the amplification factor decreased, fordecreasing the sensitivity.

The following scenarios are possible for the method:

In the operation type “MAX”, the starting state of the condition signalis “free”, while the actual state of the sensor is “covered”, i.e. thedesired state of the condition signal is “covered”. Since a false stateis shown, this must be changed by the calibration process. By manualinitiation of the calibration process, the sensitivity is decreaseduntil the comparator toggles and the desired state of the conditionsignal is reached.

In the operation type “MAX”, the starting state of the condition signalis “covered”, while the actual state of the sensor is “free”, i.e. thedesired state of the condition signal is “free”. Since a false state isshown, this must be changed by the calibration process. By manualinitiation of the calibration process, the sensitivity is increaseduntil the comparator toggles and the desired state of the conditionsignal is reached.

In the operation type “MIN”, the starting state of the condition signalis “free”, while the actual state of the sensor is “covered”, i.e. thedesired state of the condition signal is “covered”. Since a false stateis shown, this must be changed by the calibration process. By manualinitiation of the calibration process, the sensitivity is decreaseduntil the comparator toggles and the desired state of the conditionsignal is reached.

In the operation type “MIN”, the starting state of the condition signalis “covered”, while the actual state of the sensor is “free”, i.e. thedesired state of the condition signal is “free”. Since a false state isshown, this must be changed by the calibration process. By manualinitiation of the calibration process, the sensitivity is increaseduntil the comparator toggles and the desired state of the conditionsignal is reached.

1-12. (canceled)
 13. A field device having a field device electronics;and a sensor unit for process measurements, wherein: said field deviceelectronics receives measurement signals from said sensor unit; saidfield device electronics includes an evaluation unit for evaluating themeasurement signals, and means for calibrating the field device, saidmeans for calibrating the field device includes a digital element and amicroprocessor, and for calibrating the field device, said digitaladjusting element is driven by said microprocessor.
 14. The field deviceas defined in claim 13, wherein: in calibration of the field device,said digital adjusting element acts, by suitable signals, on saidevaluation unit and/or on a measurement signal production.
 15. The fielddevice defined in claim 14, wherein: said evaluation unit includes acomparator which products a condition signal (Z1) as a function of acomparison of the measurement signal (M1) with a desired value (S1), andwherein the desired value (S1) is produced by said digital adjustingelement.
 16. The field device as defined in claim 14, wherein: theaction of said digital adjusting element on the measurement signalproduction includes an amplification change.
 17. The field device asdefined in claim 14, wherein: calibration is initiated by a switchingelement which is actuated contactlessly from outside.
 18. The fielddevice as defined in claim 17, wherein: said switching element is a reedrelay, which is actuated from the outside by a magnet.
 19. The fielddevice as defined in claim 17, wherein: said switching element is a Hallsensor, which is actuated from the outside by a magnet.
 20. The fielddevice as defined in claim 17, wherein: the interface between aidactuating element and said switching element is embodied as atransmitting/receiving unit for a wireless data exchange between a idactuating element and said switching element.
 21. The field device asdefined in claim 20, wherein: the interface between said switchingelement and said actuating element is embodied as a transponderarrangement for inductive data transfer with a transmitting coil and areceiving coil, and said actuating element includes said transmittingcoil and the switching element includes said receiving coil.
 22. Thefield device as defined in claim 20, wherein: the interface is embodiedas an infrared interface or as a radio interface, and said transmittingunit is part of said actuating element and said receiving unit is partof said switching element.
 23. The field device as defined in claim 20,wherein: said actuating element is embodied as a memory card.
 24. Amethod for calibrating a field device, comprising the steps of:determining an operation type of the field device by a field deviceelectronics; determining a desired state of a condition signal (Z1) bythe field device electronics, depending on the determined operation typeand the determined desired state; changing the sensitivity of the fielddevice until a toggling of the condition signal (Z1) is obtained,wherein the desired state of the condition signal (Z1) represents theactual sensor state.